Cyclic AMP has been shown to mediate prostaglandin-induced disaggregation of blood platelets. The role of cyclic GMP in platelet aggregation is not clear. The role of cyclic GMP in blood platelet aggregation was investigated by showing arachidonic acid caused an activation of guanylate cyclase in a cell free system. This activation of the enzyme was completely inhibited by incubation under N2 and by indomethacin which inhibited the cyclo-oxygenase, the enzyme responsible for this conversion of arachidonic acid to thromboxane and various prostaglandin intermediates. These results indicated that arachidonic acid, which is released by various aggregating agents, could be converted into an intermediate which activates guanylate cyclase and thus could increase cyclic GMP. This increase then could mediate the aggregation of blood platelets.